Resonators exploiting acoustic wave propagation may currently be subdivided into three main families:
surface wave resonators, which exploit the propagation of waves localized near to the surface of a piezoelectric substrate. The transduction between the electrical signal and the acoustic waves is effected by means of electrodes in the form of interdigitated combs deposited on the surface of the substrate. Resonators are usually formed by placing a transducer with interdigitated combs T between two reflector arrays R1 and R2, obtained by applying the same potential to periodic metallizations. FIG. 1 shows a diagram of such a resonator;
Lamb wave resonators or, more generally, resonators using waves guided within a piezoelectric layer. In this type of component, the waves propagate within a wafer, or within a part of a multilayer. The transduction is here again provided by a set of electrodes in the form of interdigitated combs ESi as illustrated in FIG. 2a, the difference with respect to surface waves is that it is also possible to place one or more electrodes on the lower face of the wafer or of the piezoelectric layer, where these can be continuous EI or equally in the form of interdigitated combs EIj, as illustrated in FIGS. 2b and 2c. These configurations are notably described in the article by J. H. Kuypers, C-M Lin, G. Vigevani and A. Pisano, Intrinsic temperature compensation of Aluminum Nitride Lamb wave resonators for multiple-frequency references, Proceedings of the 2008 IEEE Frequency Control Symposium.
In order to define a resonant cavity, two configurations are commonly employed and consist in using reflector arrays composed of short-circuited interdigitated combs (like for surface wave resonators), or in bounding the wafer (or the layer acting as a waveguide) so as to cause a reflection of the waves at the edge of the wafer (or of the layer);
bulk wave resonators, which exploit waves propagating vertically within a multilayer. The transduction is provided by electrodes between which a piezoelectric layer is sandwiched in the most general case. In some alternative configurations, the two electrodes are sometimes disposed side by side on the surface of the piezoelectric layer in order to be able to be excited by a horizontal, rather than vertical, electric field.
These resonators may be subdivided into two configurations: the first is called Film Bulk Acoustic Resonator (FBAR) in which the resonator is composed solely of the piezoelectric layer. This is then acoustically isolated by the use of a Bragg mirror MR composed of alternating layers on the surface of a substrate S, as shown in the example in FIG. 3, or by the formation of a cavity of air under the piezoelectric layer Cpiezo and the control electrode ES, as illustrated in FIG. 4.
The second configuration relies on the substrate in order to take advantage of the acoustic properties of the latter. Generally speaking, this resonator is used in a harmonic resonance mode, which gives rise to its name High-overtone Bulk Acoustic Resonator (H BAR) when the substrate is thick. When the substrate is thinner, and a relatively low-order harmonic (of the order of a few units) is used, this is usually referred to as a Composite Resonator.
HBAR resonators have the advantage of allowing extremely high quality factors to be obtained at frequencies in the GHz range, as described in the article by K. M. Lakin, G. R. Kline, R. S. Ketcham, A. R. Landin, W. A. Burkland, K. T. McCarron, S. D. Braymen and S. G. Burns, Thin film resonators technology, Proceedings of the 1987 IEEE International Frequency Control Symposium, p. 371, notably when the resonator operates on a high harmonic. By a judicious choice of the ratio of thicknesses between the resonant cavity (the substrate) and the piezoelectric layer, the electromechanical coupling coefficient of a composite resonator may be adjusted as described in the article by Z. Wang, Y. Zhang and J. D. N. Cheeke, Characterization of electromechanical coupling coefficients of piezoelectric films using composite resonators, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control Vol. 46, No5, p. 1327, September 2009.
A degree of freedom for adjusting both the quality factor of the resonator and its electromechanical coupling coefficient is thus available, even if, as a general rule, increasing one amounts to decreasing the other.
These two quantities are particularly interesting for the use of resonators in radiofrequency systems where they can be used as frequency-variable impedance elements in bandpass filters, or as frequency stabilization elements in oscillators. In the first case, the electromechanical coupling coefficient defines the bandwidth attainable by a filter (the relative bandwidth is around half of the electromechanical coupling coefficient of the resonators composing it), whereas the quality factor has a direct influence on the losses of this same filter and on its selectivity. Since these two criteria are mutually exclusive, a designer tries in general to favour one to the detriment of the other. This is not however easily done using resonators of the FBAR, SAW or guided wave resonator types. This is the reason for the interest in composite resonators.
The latter however have a major defect, which in practice hinders their use: since the resonator is made to operate on one harmonic mode, the spectrum of these components has all the resonances of the unused modes, as illustrated in FIG. 5. The freedom of design previously described is not therefore used in practice for filtering applications where the resonances of the unused harmonics interfere with the operation of the filter. These components are accordingly just used for oscillator applications which are less sensitive to the unwanted resonances and where a high quality factor is imperative, as described in the article by J. Masson, G. Martin, R. Boudot, Y. Gruson, S. Ballandras, A. Artieda, P. Muralt, B. Belgacem, L. Chommeloux, Fabrication of high stability oscillators using AlN/Si high overtone bulk acoustic resonators, Proceedings of the 2007 IEEE Ultrasonics Symposium, p. 628.
On the other hand, and this is a characteristic inherent to bulk wave components, the properties of an HBAR resonator are fixed by the thicknesses of the layers composing it. It is therefore very difficult to simultaneously produce components having different properties, because that would require the local addition or removal of layers allowing the frequency differentiation. This option is, in practice, used to form two, or even four, different types of resonators on the same structure, as described in the article by Nam et al., Monolithic 1-chip FBAR duplexer for W-CDMA handsets, Sensors and Actuators A, Vol. 143, p. 162 (2008), but seldom more than this, which limits the freedom in the design of systems.